The present invention relates to a process for preparing .alpha.-methylstyrene/acrylonitrile copolymers, and more particularly to a process for preparing a copolymer having excellent transparency, heat resistance and strength in high conversions by copolymerizing .alpha.-methylstyrene and acrylonitrile with or without at least one other vinyl compound in the presence of a specific peroxide in a suspension or bulk polymerization manner.
For producing a copolymer having a heat resistance sufficient for practical use by copolymerizing .alpha.-methylstyrene, acrylonitrile and at least one of other vinyl compounds copolymerizable therewith such as styrene, methyl methacrylate, vinyl toluene and t-butylstyrene, it is necessary to employ .alpha.-methylstyrene in an amount of at least 10% by weight, preferably at least 20% by weight, of the whole monomers to be used to produce a copolymer containing at least 10% by weight, preferably at least 20% by weight, of the .alpha.-methylstyrene units.
It is proposed to carry out a suspension or bulk polymerization in the presence of an organic peroxide as a polymerization initiator such as t-butyl perbenzoate, t-butyl peracetate or di-t-butyl peroxide in a process for preparing .alpha.-methylstyrene/acrylonitrile copolymers wherein a large amount of .alpha.-methylstyrene is employed to produce a copolymer having a good heat resistance from the above-mentioned point of view. However, in a process using such initiators, it is necessary to use a large amount of the initiators regardless of the polymerization temperature. Consequently, the obtained copolymer is very low in the degree of polymerization, namely molecular weight, and is poor in utility as a molding material. On the other hand, in case of using these initiators in a decreased amount for the purpose of raising the degree of polymerization, the productivity is very low, since the so-called dead end polymerization occurs, thus a high conversion as required in industrial production is not obtained, or a very long polymerization time is required when a high conversion is desired.
Also, high conversions are not obtained by the use of a usual organic peroxide such as benzoyl peroxide, even if the polymerization temperature optimum for the peroxide is adopted.
Like this, in a conventional process for the preparation of the .alpha.-methylstyrene/acrylonitrile copolymers by suspension or bulk polymerization, the copolymers utilizable as molding materials have not been obtained, or the productivity has been very low.
It is an object of the present invention to provide a process for preparing .alpha.-methylstyrene/acrylonitrile copolymers having excellent transparency, heat resistance and strength.
A further object of the invention is to provide a process for preparing .alpha.-methylstyrene/acrylonitrile copolymers having excellent properties in high conversions in a short period of time by a suspension or bulk polymerization technique.
These and other objects of the present invention will become apparent from the description hereinafter.